1. Field of the Invention
The present invention relates to a process for fabricating graft polymerized SiO.sub.2 lithographic masks. The process can utilize either a negative or a positive resist polymer; the radiation source used to create the resist image can be any suitable precision radiation source such as electron beam, masked ion beam, focused ion beam; and, the development of the negative resist image into the lithographic mask can be achieved via a dry process in the form of plasma or reactive ion etching.
2. Description of the Prior Art
Fabrication of microcircuits or microcircuit elements requires that the feature size of structures within the circuits be on the order of micrometers or or even submicrometers. This is accomplished by utilizing a precision radiation source capable of producing such submicrometer images, such as electron beam, masked ion beam, focused ion beam, or x-ray, tailoring of the pattern defining medium (resist material) to the specific requirements of the application, and developing the pattern into the final resist structure using a method which reduces the possibility of distortion of the pattern.
Microfabrication typically utilizes a semiconductor substrate which is first coated with a layer of polymeric resist material. The polymeric resist coating is converted to a resist structure by utilizing lithographic techniques. The resist polymeric coating is exposed to patterned radiation and reacts to such radiation in a manner which results in the formation of free radicals. Depending on the type of organic structure which makes up the polymer, the molecules may crosslink with other molecules in the polymer structure or they may degrade, e.g., undergo scission. A polymer which crosslinks after irradiation is described as a negative resist polymer, while a polymer which undergoes scission after irradiation is referred to as a positive resist polymer.
After exposure to a patterned radiation, the resist is developed into the structure required. The structure may become a part of a microcircuit itself or may be used as a masking device to permit subsequent processing of the substrate to yield the desired microcircuit.
Use of a precision radiation source such as an electron beam, and the development of particular polymer and copolymer resists to accomodate such radiation source are illustrated in recent U.S. Patents and publications.
Electron beam sensitive negative resists have been described by Desai et al., in U.S. Pat. No. 4,237,208. Desai et al. utilize a silicon-containing polymer to provide sensitivity to electron beam radiation and good contrast in the developed resist. The development process utilizes a solvent or solvent mixture to remove the resist in areas which have not been irradiated. The remaining resist structure is referred to as a negative resist structure. Additional examples of electron beam sensitive negative resists are provided in U.S. Pat. No. 4,301,231 by Atarashi et al. and in U.S. Pat. No. 4,348,472 by Jagt. The latter U.S. patent provides for the use of copolymers. Copolymers have frequently been utilized to obtain resist characteristics which cannot be obtained from an individual resist polymer.
In U.S. Pat. No. 4,195,108, Gazard et al. describe the use of an initial resist polymer which is exposed to electron beam radiation, followed by a grafting reaction with an appropriate monomer to enable production of a form of copolymer with a significantly different solubility than the initial resist polymer. The grafting reaction is achieved by placing the irradiated, resist-coated substrate into a liquid solution containing the monomer to be grafted to the resist polymer. The liquid phase reaction described requires a period of one day or more, evidently to permit diffusion of the monomer into the polymeric film. Use of this process permits use of electron beam radiation with initial resist polymers which are not sufficiently sensitive to such radiation in themselves to generate a patterned resist capable of development into a resist structure.
Additional description of the above process is provided in M. Gazard et al. "Lithographic Technique Using Radiation-Induced Grafting of Acrylic Acid into Poly(Methyl Methacrylate) Films," Polymer Engineering and Science 20, 1069-1072 (1980). This 1980 article describes shorter reaction periods of from one to three hours for the liquid phase reaction.
Use of solvent to develop the patterned image from the resist polymer or copolymer is not without disadvantages. The resist polymer or copolymer often absorbs solvent, resulting in various distortions of the patterned image even after the solvent is removed. G. N. Taylor et al., "Organosilicon Monomers for Plasma-developed X-Ray Resists", Journal of Vacuum Science Technology, 19 872-880 (1981), suggest that plasma development of negative resists provides a method of avoiding resolution problems encountered with solvent development. A number of copolymer resists were prepared by codissolving a resist polymer with an organometallic monomer in a solvent and spin casting the mixture onto a substrate. After evaporation of the solvent, the mixture of resist polymer and organometallic monomer in film form was exposed to radiation in order to obtain grafting of the monomer onto active sites created by the radiation of the resist polymer. Some homopolymerization of the monomer most probably occurred simultaneously. After removal of the excess monomer, the patterned resist was developed using O.sub.2 reactive ion etching techniques.
The organometallic monomer utilized above must be of sufficiently high molecular weight and exhibit a molecular structure which provides the low volatility required during the grafting reaction. At the same time, such monomer, which remains unreacted after the grafting reaction, must be sufficiently volatile to permit removal via vacuum techniques. Thus, the organometallic monomers which meet the requirements of Taylor et al. are limited in scope.
The low volatility limitations placed on the organometallic monomer, as described above, retard the mobility of the monomer and reduce the potential for such monomer to graft to the irradiated polymeric resist. In addition, although excellent uniformity of etching was obtained by Taylor et al., a loss of 50% of the resist image thickness during the etching process was not uncommon.
It is thus desirable to develop a system capable of utilizing both negative and positive resist polymers, and a precision radiation source such as electron beam, masked ion beam, or focused ion beam. In addition, in order to take advantage of the precision images which can be created when these radiation sources are utilized, it is necessary to determine a process for development of the image which will not result in distortion of such image. The prior art has demonstrated that polymers and copolymers can be developed which are sufficiently reactive on exposure to electron beam to produce potentially useful images. Wet solvent development of these images results in distortion, however, which does not permit full utilization of the precisely defined image. Attempts have been made to graft organometallic molecules onto the resist backbone polymer in order to provide a resist which can be dry developed via plasma or reactive ion etching. However, there has not been a satisfactory process achieved which produces a resist image which can be dry developed without substantial loss of the resist or damage to the resist polymer during development.